1. Field of the Invention
The present invention relates to a data readout method, a data readout device, and an optical disk, and in particular, to a data readout method and a data readout device for reading out data recorded in an optical disk, and an optical disk used in the data readout device.
2. Description of the Related Art
In the related art, when reading data recorded in read-only optical disks, such as a CD (Compact Disc), a DVD (Digital Versatile Disc), a Blue-ray Disk, and a HD-DVD, the readout power can be any value within a dynamic range of a detection circuit, and there is no damage to a readout layer formed only of a reflective layer.
In addition, in the related art, when reading data recorded in writable optical disks, such as a CD-R (CD-readable), a CD-RW (CD-rewritable), a DVD-R (DVD-readable), a DVD-RW (DVD-rewritable), a DVD+R (DVD+readable), a DVD+RW (DVD+rewritable), and a DVD-RAM, it is required to use a readout power sufficiently low so as not to eliminate or destroy mark series on a recording layer.
Further, in the related art, for either of the two types of optical disks, usually the readout power can be arbitrarily adjusted by a readout device.
In recent years and continuing, it is required that the optical disks have large capacities. So far, various methods have been proposed to realize large capacity optical disks, for example, one of them is a super resolution readout method. In the super resolution readout method, marks and spaces shorter than the diffraction limit of an optical system are formed on an optical disk to increase recording density and capacity of the optical disk. In addition, a light beam of high power is condensed onto a readout layer made of phase-change materials (this readout layer is referred to as “a super resolution readout layer”) to increase the temperature of a portion of the light-condensed area of the readout layer, thereby changing the optical properties (optical constant) of this area. Consequently, it is possible to read out data exceeding the diffraction limit. Currently, read-only optical disks capable of such super resolution readout are in the process of being placed into practical application.
Below, for convenience, a recording density not exceeding the diffraction limit is referred to as a “usual density”, and readout of data recorded at the usual density is referred to as “usual readout”.
Below, an optical disk capable of the super resolution readout is explained.
FIG. 9A through FIG. 9C are schematic cross-sectional views illustrating examples of configurations of an optical disk capable of the super resolution readout.
The optical disk shown in FIG. 9A has a stacked layer structure including a protection layer and a readout layer on a substrate. For example, the protection layer is formed from ZnS—SiO2, and the readout layer is formed from a phase-change material like AgInSbTe.
The optical disk shown in FIG. 9B is disclosed in “Jpn. J. Appl. Phys., 42, pp 995-996 (2003)” (hereinafter referred to as “reference 1”), which has a stacked layer structure including a protection layer, a readout layer, and a recording layer on a substrate. For example, the protection layer is formed from ZnS—SiO2, the readout layer is formed from GeSbTe, and the recording layer is formed from AgOx.
The optical disk shown in FIG. 9C is disclosed in “ISOM Technical Digest, pp 66-67 (2004)” (hereinafter referred to as “reference 2”), which has a stacked layer structure including a protection layer, a readout layer, and a recording layer on a substrate. For example, the protection layer is formed from ZnS—SiO2, the readout layer is formed from GeSbTe, and the recording layer is formed from PtOx or W—Si.
A laser beam is incident on the optical disk shown in one of FIG. 9A through FIG. 9C from the substrate side or the side opposite to the substrate to perform data recording and data super resolution readout. During data recording operation, the laser beam is irradiated onto the optical disk at a recording power, thereby forming recording marks on the recording layer. The mechanism of forming the marks on the recording layer depends on the materials of the media. For example, for the AgInSbTe layer of the optical disk shown in FIG. 9A, the recording marks are formed by shape changes; for the AgOx layer or the PtOx layer of the optical disks shown in FIG. 9B and FIG. 9C, the recording marks are formed by generation of metal fine particles and bubbles; for the W—Si layer of the optical disk shown in FIG. 9C, the recording marks are formed by chemical reactions between Si and W. Since the recording marks are formed only in locally heated regions within the laser spot, the recording marks can be formed to be shorter than the diffraction limit of the optical system.
In addition to the above recordable optical disks, a read-only super resolution readout optical disk is also proposed, which is fabricated by forming the above mentioned readout layer on a ROM-type substrate, and the ROM-type substrate is molded with a stamper obtained by high density mastering through Deep UV (Ultra Violet) exposure or EB (Electron Beam) exposure.
In a magneto-optic (MO) recording medium, the super resolution readout is realized based on another principle. For example, in a DWDD super resolution readout disclosed in Japanese Laid-Open Patent Application No. 2001-52386 (hereinafter referred to as “reference 3”), since the ratio of amplitudes of readout signals varies slowly near a super resolution readout power, and does not possess linearity, amplitude variations with respect to a long mark series and a short mark series are monitored, and the optimum readout power is determined when the above amplitude variations become equal.
Japanese Patent Gazette No. 3292773 (hereinafter referred to as “reference 4”) discloses a technique in which the optimum readout power is determined when a signal amplitude of pre-recorded super resolution readout reproduction pits becomes the maximum.
Japanese Laid-Open Patent Application No. 2003-16653 (hereinafter referred to as “reference 5”) discloses a technique in which an error between the current readout power and the optimum readout power is detected based on the magnitude of equalization during adaptive equalization, and the readout power of the laser beam is controlled so that the error becomes zero, namely, the error of a waveform equalization coefficient is minimum.
Japanese Patent Gazette No. 3762922 (hereinafter referred to as “reference 6”) discloses a technique in which plural readout signals are produced by modulating the recording light quantity and readout light quantity, and a combination of an optimum recording light quantity and an optimum readout light quantity minimizing an error rate is selected from the plural readout signals; namely, the error rate is minimum.
However, since all of the above techniques are based on detection of the variation of signal amplitudes, or detection of modulated readout signals, because the signal amplitude is small (that is, the sensitivity is low), it is difficult to calculate the optimum readout power, namely, a power that is near a maximum CNR (Carrier-to-Noise Ratio) value of the readout signals and not resulting in degradation of the readout film of the recording medium. Here, the CNR value of the readout signals is an index of a super resolution effect, namely, associated with a largest increase of the signal intensity (carrier level C). Particularly, when a readout film formed of the phase-change materials is used in super resolution readout, the signal amplitude variation is small. Since pit (mark) lengths associated with small signal amplitudes are involved in modulation using PRML (Partial-Response Maximum Likelihood), like the HD-DVD, it is difficult to ensure detection accuracy.
When using a readout device to reproduce the optical disk capable of super resolution readout, first, in order to identify that the optical disk in use is capable of super resolution readout, it is necessary to read out data in a control data area, in which control data are recorded at the usual density. In this case, it is required that the readout power be sufficiently low so as not to induce the super resolution readout. Further, when it is identified that the optical disk in use is capable of the super resolution readout, in order to read a user data area of the optical disk in which user data are recorded at a recording density exceeding the diffraction limit, it is required that the readout power be appropriate so that a readout layer not be destroyed, and the smallest marks (pits) can be read with high signal intensity.
When the readout power used in the super resolution readout is lower than the optimum readout power, the incident light can hardly induce changes of the optical properties in a region in a light spot of the condensed light, and thus only data recorded at the usual density can be read.
When the readout power used in the super resolution readout is higher than the optimum readout power, the incident light induces a too large region in the light spot of the condensed light in which region changes of the optical properties occur, and thus the super resolution readout cannot be realized, or the number of repeated readings is noticeably reduced due to degradation of the readout layer and damage to the recording marks or pits.
In other words, it is necessary to use two levels of readout power having different values for the optical disk which supports the super resolution readout. The readout devices of the related art do not have such a configuration, and are unable to adjust to obtain the optimum readout power.
As disclosed in Japanese Patent Gazette No. 3571624 (hereinafter referred to as “reference 7”), with an OPC (Optimum Power Control) technique of the related art, one may attempt to utilize an increased degree of modulation of a readout signal amplitude; however, in order to calculate the optimum readout power, after detecting the signal amplitude, from the variation of the signal amplitude it is difficult to calculate the optimum readout power with high accuracy when the readout signal is small, as in the super resolution readout.